A. Field of the Invention
The present invention relates to binders and other articles of manufacture having sheets of plastic bonded together and, in particular, a method for ultrasonically sealing a plastic film during the manufacture of a binder.
B. Prior Art
The manufacturing process of polypropylene clear view binders involves ultrasonically sealing and cutting sheets of plastic at the edges of the binders. Waste material is then stripped away from the edge. U.S. Pat. No. 5,476,570, which issued on Dec. 19, 1995 to Widmann and which is incorporated by reference herein, discloses a method for manufacturing folders or binder jackets that utilizes ultrasonic welding.
An important aspect of a commercial binder manufacturing operation is the speed with which binders can be produced. While high rates of production are desired, production is typically limited by the need to ensure that the plastic sheets are well-sealed, that the cuts are clean, and that the edges are smooth after the waste material is stripped. In particular, problems that occur at high production rates of greater than about 20 binders per minute include ragged, bumpy edges, blemishes and stringiness.
There is therefore a need for an improved method of ultrasonically sealing a plastic film composed of polypropylene that can be performed at higher production rates and yield products that are free of defects such as ragged or bumpy edges, blemishes, or stringiness.